1) Field of the Invention
The present invention relates to optical fiber devices adapted to be suitably used in coupling optical fibers optically or in branching light propagating through optical fibers for optical branching. Further, the present invention relates to optical monitors and optical monitors.
2) Description of the Related Art
In Internet data centers (IDCs), etc., a plurality of server computers are used and their data input/output ports are utilized by interconnecting them through multi-mode fibers. In the existing circumstances, in order to switch connections depending on the service to be provided, the switching is performed manually and therefore there is a demand for provision of a technique, which enables automatically switching connections of multi-mode optical fibers. More specifically, there is a demand for an optical switch for switching connections among multi-mode optical fibers.
As a method for configuring such an optical switch, for example, as an optical switch 100 illustrated in FIG. 18, there is a method in which a single optical fiber 111 and N optical fibers 112-1 to 112-N are opposed to each other and the single optical fiber 111 is mechanically moved to switch connections. Namely, by mechanically moving the optical fiber 111 through an actuator 117, a single optical fiber 112-i (i=i to N) to be optically coupled to the optical fiber 111 can be selectively switched, among the N optical fibers.
In order to realize the optical switch 100 as described above, there is required an optical-fiber optical coupling technique which facilitates mechanically moving the optical fibers 112-1 to 112-N as well as attenuating back-reflected-light and enables high-efficiency optical coupling, in order to switch among the optical fibers 112-1 to 112-N opposed to the optical fiber 111.
In order to realize the optical coupling between the optical fibers 111, 112-i in the aforementioned optical switch 100 illustrated in FIG. 18 by using multi-mode optical fibers, utilization of a butt-joint coupling system illustrated in FIG. 22, which will be described later, is generally conceived, in view of the fact that reduction of a loss is facilitated.
Further, when the aforementioned multi-mode optical fibers are interconnected, there is also a need to provide a function of monitoring the optical coupling efficiency based on the switching of optical fibers as described above, in order to maximize the coupled light power. Namely, based on the result of monitoring using the monitoring function, the position of the optical fiber 111 is adjusted by an actuator such that the coupled light power is maximized.
For example, as exemplified in FIG. 19, when the optical fiber 111 and the optical fiber 112-i are coupled to each other, there are provided, on the respective optical fibers 111, 112-i, in-line power monitors 81, 82 for monitoring the light power coupled to the respective optical fibers 111, 112-i, and there is also provided a controller 116. Thus, this controller 116 controls the actuator 117 based on the result of monitoring through the in-line power monitors 81, 82 to adjust the position of the optical fiber 111 such that the coupled power is maximized
Further, in order to monitor the light output using the aforementioned power monitors 81, 82, it is common practice to branch light propagated through the optical fibers 111, 112-i for extracting light to be monitored. Thus, there is also a demand for an optical branching technique, which brings about a low branching excess loss in performing such optical branching.
Technique of Optically Coupling Optical Fibers
As the aforementioned technique of optically coupling optical fibers, there are lens coupling systems 101, 102, for example, as illustrated in FIG. 20(a) and FIG. 20(b). In the lens coupling system illustrated in FIG. 20(a), single-mode optical fibers 111s, 112s opposed to each other are optically coupled to each other through two condenser lenses 121′, 122′ having substantially the same focal length. Namely, light emitted from the single-mode optical fibers 111s, 112s is converted into parallel light rays by the condenser lenses 121′, 122′ and then is converged again and coupled. In FIG. 20(a) and FIG. 20(b), the reference character 113 designates light beams.
In the lens coupling system 101, the distance between the lenses 121′, 122′ can be arbitrarily set while the lenses 121′, 122′ and the single-mode optical fibers 111s, 112s are arranged such that the end face of the single-mode optical fiber ills is arranged at the focal length of the lens 121′ and the end face of the single-mode optical fiber 112s is arranged at the focal length of the lens 122′. In the case where the lens coupling system 101 is used for optical coupling between the single-mode optical fibers 111s, 112s having the same configuration, the converged-light spot size of light 113 at the beam waist is identical with the divergent angle of diffraction, so as to allow realization of a low-loss coupling system.
The lens coupling system 102 illustrated in FIG. 20(b) is disclosed, for example, in the following patent literatures 1 to 3. In the lens coupling system 102, similarly, two optical fibers 111, 112 opposed to each other are interconnected through two condenser lenses 121, 122. However, the focal length of the lens 121 is f1 and the focal length of the lens 122 is f2, which is different from f1. The lens 121 is arranged at a location associated with the focal length of the optical fiber 111 and the lens 122 is arranged at the focal length of the optical fiber 112, while the distance between the lenses 121, 122 is set to f1+f2. Here, 111c and 112c are the cores of the optical fibers 111, 112, respectively.
When the lens coupling system 102 is used for interconnecting single-mode optical fibers having the same configuration or for interconnecting multi-mode optical fibers having the same configuration, the converged-light spot size of light 113 near the end faces of the optical fibers 111, 112 is in conformity with the incident/output angle of light, so as to allow realization of a low-loss coupling system. Herein the statement “the lens coupling system is used for interconnecting multi-mode optical fibers having the same configuration” refers to the case where the core diameter a1 of the optical fiber 111 and the core diameter a2 of the optical fiber 112 are equal, the largest tilt angle α1 of light emitted from the core 111c of the optical fiber 111 and the largest tilt angle α2 of light emitted from the core 112c of the optical fiber 112 are equal in FIG. 20(b) and the lens coupling system is used for interconnecting these multi-mode optical fibers. Further, in the case where the distance between the lenses in the lens coupling system 101 is set to 2f, there is to be obtained the lens coupling system having the same configuration as the coupling system 102 in which the focal lengths f1, f2 of the two lenses are identical in the length f and the largest tilt angles α1 and α2 of light emitted from the cores of the two optical fibers are equal.
When the lens coupling system 102 is used for optically coupling optical fibers (including multi-mode optical fibers) having different core diameters, a low-loss coupling system can be realized in the case where the following conditions #1 and #2 are satisfied at the same time, wherein the divergent tilt angles of light incident on and emitted from the optical fibers 111, 112 are defined as α1 and α2, respectively, as illustrated in FIG. 20(b). Here, the diameter of the core 111c of the optical fiber 111 is defined as a1 and the diameter of the core 112c of the optical fiber 112 is defined as a2. The focal length of the condenser lens 121 is defined as f1 and the focal length of the condenser lens is defined as f2.
Condition #1: the ratio between f1 and f2 is equal to the ratio between a1 and a2.
Condition #2: the ratio between tan (α2) and tan (α1) is equal to the ratio between a1 and a2.
A lens coupling system 102A in FIG. 21(a) illustrates an example where the lens coupling system 102 in FIG. 20(b) is used for interconnecting optical fibers having different core diameters and a lens coupling system 102B in FIG. 21(b) illustrates an example where the lens coupling system 102 is used for interconnecting optical fibers having the same core diameter. When the diameters of the cores 111c, 112c are equal as in the lens coupling system 102 illustrated in FIG. 21(b), the focal lengths of the two condenser lenses 121, 122 are equal and the distance between the lenses is twice the focal length of the lenses, based upon the aforementioned conditions #1 and #2.
The lens coupling system illustrated in FIG. 21(b) can be applied for interconnecting optical fibers having the same configuration either when the optical fibers are single-mode optical fibers or when the optical fibers are multi-mode optical fibers, and the distance between the lenses 121, 122 will be fixed to f1+f2.
Further, as another optical-fiber coupling system, there is a butt-coupling system 103 as exemplified in FIG. 22. In the butt-coupling system 103, the end faces 111e, 112e of two optical fibers 111, 112 are opposed to each other and butted against each other to optically couple them. When the butt-coupling system 103 is utilized for optically coupling single-mode optical fibers having the same configuration or multi-mode optical fibers having the same configuration, a low-loss coupling system can be realized in the case where there is no space (gap) G between the butted optical fibers 111, 112 and no reflection at the end faces 111e, 112e. 
Optical Branching Technique
It is conceivable to apply an optical-fiber coupler described in the following patent literature 4 as the optical branching technique for monitoring the condition of coupling between fibers. The patent literature 4 discloses an optical-fiber coupler 4 as illustrated in FIG. 23. The optical coupler 104 illustrated in FIG. 23 is configured by bringing two optical fibers 91, 92 close to each other, thermally fusing and then drawing them.
The optical-fiber coupler 104 is configured such that the cores 91c, 92c are brought close to each other in the fused/drawn portion 93. Namely, when an AA′ cross section 94A of the fused/drawn portion 93 is compared with a BB′ cross section 94B of the other portion, it can bee seen that the cores 91c, 92c are relatively close to each other. Therefore, by coupling evanescent waves between the cores 91c, 92c at the fused/drawn portion 93, light from one of the cores is coupled to the other core, in such a manner as to branch the light. The longer the fused/drawn portion 93 becomes, the larger the amount of coupled light becomes and, therefore, the branched power becomes.
When the-optical fiber coupler 104 illustrated in FIG. 23 is utilized for causing light in single-mode optical fibers or single-mode wave guides to branch off, couplers with various branching ratios and low excess losses can be realized.
(Patent Literature 1) Laid-Open (Kokai) HEI 01-177003
(Patent Literature 2) Laid-Open (Kokai) HEI 08-15564
(Patent Literature 3) Laid-Open (Kokai) 2002-55276
(Patent Literature 4) Laid-Open (Kokai) 2001-324644
However, when the aforementioned optical-fiber coupler 104 illustrated in FIG. 23 is configured using multi-mode optical fibers, a difficulty arises in controlling the coupling of required evanescent waves, with the result that optical branching with stable branching ratios is unable to be realized. Therefore, there is a problem that the technique of the aforementioned optical-fiber coupler 104 illustrated in FIG. 23 cannot be utilized for optically branching multi-mode optical fibers.
On the contrary, as an optical system 105 illustrated in FIG. 25, for example, it is conceivable to arrange an optical branching member such as a half mirror 114 which reflects a part of the incident light power and passes a part thereof, between the condenser lenses 121′, 122′ in the lens coupling system 101 illustrated in FIG. 20(a), in order to cause light to branch off. Further, when light reflected at the half mirror 114 is received by a photoelectric conversion device 115 through a condenser lens 123 and an optical fiber 113, etc., light propagated between the optical fibers 111, 112 can be monitored. In FIG. 24, the same reference numerals or symbols as those in FIG. 20(a) designate substantially similar components.
However, if the lens coupling system 101 illustrated in FIG. 20(a) is used for interconnecting multi-mode optical fibers, this will increase the loss as will be described later. Therefore, there is a problem that the configuration in which the half mirror 114 is provided in the lens optical system 101 of FIG. 20(a) is difficult to use for multi-mode optical fibers.
A lens coupling system 101m illustrated in FIG. 25 is configured by applying the aforementioned lens coupling system 101 illustrated in FIG. 20(a) for interconnecting multi-mode optical fibers. As illustrated in FIG. 25, when multi-mode optical fibers 111m, 112m are coupled or optically interconnected by means of the lens coupling system 101, the incidence angles of light ray components 133, 134, for example, incidence on the optical fiber 112m are greater than the angles of total reflection of light between the core and the clad of the optical fiber 112m. Therefore, the light ray components are not coupled to the optical fiber 112m (namely, they can not be propagated through the core of the optical fiber 112m while being totally reflected). This brings about losses when the optical fibers 111m, 112m are coupled. Further, the condition of modes propagating through the optical fibers 111m, 112m changes, which causes the light components 133, 134 constituting light beams to occur or disappear with time. This causes noise, consequently reducing the SN ratio (signal-to-noise ratio).
Also, it is conceivable to interpose a half mirror 114 as illustrated in FIG. 24 and a photoelectric conversion device 115 between the condenser lenses 121, 122 in the lens coupling system 102 illustrated in FIG. 20(b). However, since the distance between the condenser lenses 121, 122 is short, this restricts the sizes of optical members such as the half mirror 4 which can be installed between the lenses, which cause restrictions on the design.